Automatic metal protecting apparatus and method

ABSTRACT

When a metal strip passing through a furnace is overheated, the furnace is automatically shut down in a special way to prevent outside air from rushing in to cause strip oxidation in critical furnace areas. The fuel-air mixture in the furnace is gradually reduced to a low rate which is substantially above zero, and then continues at that low rate for a definite period of time. Also, a non-oxidizing gas such as nitrogen is gradually introduced into the furnace at a gradually increasing rate, reaching its full flow after a definite period of time, while maintaining positive pressure in the furnace chamber. The nitrogen flows for a definite period of time at full volume, and then the gas-air fuel mixture is cut off, still maintaining positive pressure in the furnace chamber. This avoids leakage of air into the furnace; further, exhaust stack dampers, which are initially open, as required automatically to maintain positive furnace pressure, are gradually and automatically brought to a closed position to assist in preventing outside air from being drawn into the furnace chamber.

llnited States Patent [191 Gildersleeve Aug. 6, 1974 AUTOMATIC METALPROTECTING APPARATUS AND METHOD [76] Inventor: William E. Gildersleeve,44 Taylor Rd., Conshohocken, Pa. 19428 22 Filed: Oct. 26, 1973 [21]Appl. No.: 410,220

3.72.1,520 3/1973 Bloom 432/8 Primary Examiner-John J. Camby [57]ABSTRACT When a metal strip passing through a furnace is overheated, thefurnace is automatically shut down in a special way to prevent outsideair from rushing in to cause strip oxidation in critical furnace areas.The fuel-air mixture in the furnace is gradually reduced to a low ratewhich is substantially above zero, and then continues at that low ratefor a definite period of time. Also, a non-oxidizing gas such asnitrogen is gradually introduced into the furnace at a graduallyincreasing rate, reaching its full flow after a definite period of time,while maintaining positive pressure in the furnace chamber. The nitrogenflows for a definite period of time at full volume, and then the gas-airfuel mixture is cut off, still maintaining positive pressure in thefurnace chamber. This avoids leakage of air into the furnace; further,exhaust stack dampers, which are initially open, as requiredautomatically to maintain positive furnace pressure, are gradually andautomatically brought to a closed position to assist in preventingoutside air from being drawn into the furnace chamber.

19 Claims, 3 Drawing Figures PATENTED RUG 61914 SHEET 2 BF 3 ZONE 1 ZONE2 PILOT 2 QUENCH g PILOT N QUENCH FURNACE STRIP TEM PERATU RE CONTROLLERBRIEF DESCRIPTION OF THE INVENTION The present invention relates tostrip metal heating, and more particularly to a method and apparatus forcontrolling a strip heating furnace to prevent harm to the strip due tooverheating or oxidation.

In the continuous heating of metal strip for annealing and galvanizingor other purposes, the strip is pulled through a furnace chamber infront of open burners. When, for some reason, the strip slows down orstops, the burners have usually been quickly throttled to a minimum orshut off quickly in order to prevent damage to the strip by oxidation,overheating, or otherwise.

At the same time, it has been founddesirable that a cooling gas of anon-oxidizing type be introduced into the furnace to reduce thetemperature of the strip and of the furnace refractory lining below avalue that causes harm to the strip. Further, in performing thisoperation, it is highly desirable to prevent excessive oxidation of thestrip surface. When the strip is to be galvanized, substantially alloxidation must be prevented if possible.

The U.S. Patent to Cope et al U.S. Pat. No. 3,396,951, granted Aug. 13,1968, discloses a purging means for a gas-fuel system for a continuousstrip heating chamber which has a control system that is responsive to areduction in rate of movement of the strip for immediately shutting offthe fuel supply and instantaneously introducing a non-oxidizing gas intothe heatin g chamber, in an endeavor to prevent oxidation of the stripsurfaces. We have found, however, that in many furnaces the simultaneousshutting off of the burners and the introduction of a cooled gas such asnitrogen or fuel gas into the heating chamber causes a sudden cooling ofthe heated chamber, with consequent sharp reduction in chamber pressureto below atmospheric pressure, causing outside air to be drawn into thechamber, through small unsealable openings in the furnace, tending tocause oxidation of the strip. This strip oxidation sometimes,particularly in the case of rather thin gauge strip, causes stripburn-offs and tearing of the metal upon subsequent start-up.

It is frequently the practice in the continuous heating of metal stripto pull it vertically downwardly through a furnace chamber having openburners, in which event the combustion products from the burners comeinto contact with the surfaces of the strip, and then to conduct thestrip through a long horizontal treatment chamber provided with indirectheating tubes. The horizontal chamber contains a non-oxidizingatmosphere, such as nitrogen, for example. When the strip heating lineis shut down for any reason, it is important to maintain a properprotective atmosphere in thehorizontal heat treating section of theprocess.

Accordingly, any sudden reduction of pressure in the direct firedportion of the furnace, when communicated to the connected horizontalheat treating chamber, immediately causes a sudden reduction of pressureto below atmospheric pressure in the horizontal heat treating chamber,inducing outside air to leak through small unsealable openings in thehorizontal heat treating chamber to contaminate the protectiveatmosphere, causing oxidation and damage to the metal.

This is of particular importance, because such horizontal heat treatingchambers are often very long, such as 500 feet in length, for example,and contain a substantial amount of valuable heat treated strip. Whensuch strip is damaged by oxidation as a result of contamination of theprotective atmosphere, substantial production losses are incurred. Inaddition, before production can be resumed, the chamber must becompletely purged of all oxygen.

It is necessary when the moving strip is brought to a stop, to cool thechamber of the vertical furnace so that the strip in this section willnot become oxidized to the extent that there is an appreciable loss incross section or that it loses strength from becoming too hot. Theinitial cooling while the strip is decelerated to a stop is provided bya non-oxidizing cooling medium in the lower portion of the furnace. Theupper portions are cooled with air, an oxidizing medium.

Accordingly, it is particularly important to avoid a sudden reduction ofpressure in the direct fired portion of the furnace, and in otherportions of the strip heating production line.

It is accordingly an object of the present invention to provide acontrolled system for a strip heating furnace which operates effectivelyto prevent damage to the strip upon overheating of the strip for anyreason.

It is a further object of this invention to control the temperature andatmosphere of a strip heating furnace so that the strip is at all timesproperly heated and kept in a bright condition, except for a shortsection on a line stop.

Modern strip heating furnaces have been developed to such an extent thatthe strip can be heated rapidly to a predetermined temperature. One typeof a difficulty occurs when the strip is slowed down below a minimumoperating speed, or is stopped. The furnace temperature must be reducedrapidly to prevent the strip from being burned through, or heated tosuch an extent that it breaks of its own weight, or tears or breaks whenit is subjected to the stresses of the startup operation. In addition,the atmosphere of the furnace must be controlled so that excessiveoxidation does not take place.

It is accordingly another object of this invention to provide anautomatic shutdown system for a strip heating line, which automaticallyreduces the amount of heat that is applied to a strip which is in dangerof overheating, which prevents the introduction of oxidizing substanceswhich might be harmful to the strip, which maintains a positive pressurein the treating area throughout the period of shutdown and affirmativelyeliminates the sudden pressure drop which usually accompanies thecooling of the strip and of the area within the direct fired section ofthe furnace chamber, and which affirmatively maintains a positivepressure to eliminate the leakage of air into any area where it candamage the surfaces of the strip.

Other objects and advantages of this invention, including the simplicityand economy of the same, and the ease with which it is adapted toexisting strip heat treating lines, will further become apparenthereinafter and in the drawings.

DRAWINGS Referring to the drawings: FIG. 1 is a diagrammatic view inside elevation showing a portion of a strip heating furnace and aconnected horizontal heat treating chamber illustrating certain featuresof this invention;

FIG. 2 is a schematic diagram showing the arrangement of the automaticcontrol means comprising one embodiment in accordance with thisinvention, and

FIG. 3 is an electrical diagram illustrating the time sequence ofoperations in accordance with one embodiment of this invention.

Referring to the specific form of the invention shown in the drawings,specific terms will be used in this specification for the sake ofclarity, but it should be understood that these terms are not intendedto limit the scope of the appended claims.

Turning now to FIG. 1 of the drawings, the strip 1 is passed over guiderolls 2 and 3 to travel downwardly in a vertical path through furnace 4.The furnace may be of the type shown in the U.S. Pat. No. 2,869,846 toBloom or the U.S. Pat. No. 3,320,085 to Turner, for example. It isprovided with radiant cup-type burners 5 which face the strip and firedirectly into the furnace chamber. Products of combustion rise in thechamber and are exhausted through ducts 6 at the top of the furnace andare conducted to the stack 6'.

The furnace through which the strip travels is maintained at a minimumcritical temperature, and the fuelair ratio is controlled to provide thenecessary reducing character of the gases (products of combustion) foreffecting proper heating and final strip clean-up. The fuel-air ratio ofthe furnace is regulated to provide a slight excess of fuel so thatthere is no free oxygen in the furnace atmosphere, and so that there arefrom 3 percent to 6 percent combustibles in the form of carbon monoxideand hydrogen.

When the strip leaves the furnace, it has a clean surface suitable forproducing an adherent coating that must be protected. To this end, uponleaving the furnace, the strip passes through a throat 7, around abottom roll 10 and to and through a horizontal treating chamber 9 thatis filled with a protective atmosphere which can be neutral or reducing.The delivery end or snout 23 of this horizontal treating chamber 9 isbelow the level ofthe molten zinc 11 in pot 12. Suitable guide rolls 13are provided to guide the strip through the horizontal treating chamber9. Horizontal treating chamber 9 is preferably insulated and may haveconventional provisions for heating and/or cooling so that the strippassing through it will be brought to the desired coating temperature.Metals other than zinc, such as aluminum for example, may be used in thepot 12.

The strip may be heated in horizontal treating chamber 9, using radianttubes 14. A protective gas, such as nitrogen or a mixture of nitrogenwith a small percentage of hydrogen, is usually introduced into thechamber 9, preferably at various points designated in the drawing asHNX.

The number 15 designates a temperature sensing device, conveniently ofthe optical pyrometer type, which is arranged to sense continuously thetemperature of the strip 1 as it passes around the bottom roll 10. Itssignal is electrically connected to operate important sequences, as willfurther be described in connection with F IGS. 2 and 3.

The top of the furnace chamber is slightly open at 16 to provide anentry slot for the strip 1. Air bars 20, 20 are provided above and toeach side of the opening 16. Each air bar 20 is an elongated pipe andhas a plurality of spaced air jets which are aimed obliquely downwardlyinto the entry slot 16 in a manner to impede the flow of gases upwardlyfrom the furnace chamber, outwardly through the opening 16.

The number 21 represents a damper in duct 6, which normally controlsfurnace chamber pressures and is opened and closed by a damper operator,usually in the form of a cylinder 22. The damper 21 is centrally mountedin duct 6, and swings adjustably about its shaft 6(a) and thus controlsfurnace chamber pressures. It is opened and closed by the air cylinder22 operating the shaft 6(a) through a mechanical linkage. Damper 21increases furnace back pressure when it is closed. It is, however,incapable of forming a perfect seal when closed, since leakage space ispresent all around the edge of the damper in order to allow forexpansion.

There are many opportunities for air to leak into furnace 4 orhorizontal treating chamber 9 in response to a sudden reduction offurnace chamber pressure. Typical leakage areas include the hearth rollshafts 13 (which extend through chamber 9), access doors, threadingdoors, bolted and clamped connections, the snout 23 which corrodes incontact with molten zinc, and light and sight ports.

Heretofore, it has been considered desirable, upon encountering astoppage of the strip, to shut off the burner fuel and air of the directfired vertical furnace and simultaneously introduce a cold neutral gassuch as nitrogen, in order to protect the metal of the strip fromoverheating and oxidation. However, it has now been found that suchaction in some cases results in a sudden reduction of furnace chamberpressure, causing air to be drawn strongly into the furnace chamber,oxidizing the strip. Further, such sudden reduction of furnace chamberpressure communicates itself through slot 7 to the horizontal duct 9,which is intended to hold a protective atmosphere, but which is invadedby outside air through the leakage points heretofore mentioned.

In accordance with this invention, the timing and rate of introductionof the cooling gas and of the fuel and air to the burners are controlledin a critical manner to maintain a positive pressure in the furnacechamber throughout the entire shutdown procedure. This is accomplished,in essence, by avoiding a simultaneous introduction of cooling gas andshutting off of the fuel and air to the burners. Preferably, the coolinggas is only gradually introduced, with gradual increase of rate ofintroduction and the flow of fuel and air is reduced gradually to apredetermined low rate, which is maintained at a constant rate for aperiod of time. After the cooling gas flow has reached a steady rate andhas continued at such rate for a definite period of time, the flow offuel and air is shut off.

Such a procedure maintains positive pressure in the furnace chamber andin the horizontal treatment chamber 9 as well a factor of considerableimportance because a very substantial length of finished strip ismaintained in the chamber 9, and would all be subjected to damage byleakage of air into the chamber 9.

As will be apparent from FIGS. 2 and 3 of the drawings, which show onepreferred way of carrying the invention into effect, a relay isincorporated into the operating circuit of the equipment which controlsthe running of the strip through the furnace. This relay is energizedwhen the line is in the Run" condition as opposed to a .log" or Threadcondition. When this relay is energized the strip line is intended torun at normal production speeds under control of the line opera- I01,and the furnace IS started Lin anautomatically 56- Valve A4 Nitrogenquench (bottom of Furnace Closed 7' quenced cycle. When this relay' isde-energized the furnace is shut down in an automatically sequenced.cycle. Valve Bl Zone i. Burner Air Shut-Off Closed Valve B2 Zone 1,(Fuel) Gas ShutOff Closed Valve V Re is a re 'ulatin i valve which main-It IS an inherent characteristic of a furnace heating tuins ii progerfuel-tfi-air hint.

Valve B3 Zone 2. Nitrogen for hurner cooling Open Stnp .that there is alow Speed point at which i i 1m Valve Cl Zone 2. Shut-Off valve forfuel-air Closed practical to operate, at which speed even the minimummixture. heat input tends to cause strip overheating. This speed pointvaries with the gauge or thickness and width of Turningnow to FIG. 3,which is a simplified electristrip being processed. A contact isincorporated into cal sequencediagram for a furnace of a configurationthe control system of the line, which contact is autoas shown in FIG. Iand with valves as shown in FIG. 2: matically closed at speeds below thespeed at which the In this diagram 011 line 19 a relay (llne P lightestgauge strip can be processed. At such low lay) is shown. This relay ISprincipally operated by the speeds the line is automatically stopped andthe furstrip moving mechanism and is energized when the nace is shutdown in an. automatically sequenced cycle, l5 strip moving n m y Thedeenerglzmg 0f thlS relay when the strip temperature sensor 15 detectsan overcauses the moving strip to come to a stop. temperature conditionas the. strip leaves the direct A l y n P ay) 1S Shown m lme fi d ti fth f 23; which when energized causes the furnace stop se In describingthe automatic controls, a two-zone furquence to be initiated. When relayF.S.R. is deenernace has been selected for convenience; itwill beapgizecl, the furnace start sequence is initiated. preciated that thisinvention is-applicable to furnaces The furnace stop relay in line 23has several contacts having any number of zones. F.S.R. which are in thefollowing positions with the line Turning now to FIG. 2, a simplifiedcombustion and stopped. (The relay is energized with the line stopped).cooling system is shown. The valvesshown are typically supplied withoperators such as cylinders or diaphragm operators or other types whichare designed to operate Line (1 5), (9), & (21) Closed (these all closeon line stoppage). alvalv e iltlt a definitz:l fixled o controllablespeed. Aln Line (3), (8L (22) Open (these anopen on e ectrica y operateso enoi type va ve is typica y line stoppage) used to cause the valvesto function. In this diagram, 7 i A A2 g; g temperature comm: Thefurnace sequence. timer controls are in the folva ves wit operators erom t e temperature contro lowing positions with the line Stopped.system. Means are provided to cause this valve to move to apredetermined m nimum flow position by the re- S4 (Line (Line 8), movalof an electric signal regardless of the temperas-2 (Line 13), turesignal. All other valves A1,A3-,A4, B1, B2, B3and (Lme 22) tggz gyf C1are arranged so that, when an electrical signal is ap- (Line 3), (Line4) plied the valves open at a controlled rate, and when the (Linc l d hl l t n d S-8 (Line 21) Open (these all open on sigtna IS mterrupte t eva ves c ose at a con ro e line stoppage) ra e.

Referring again to w t e line pp the When the strip line is in the Run"condition the furfurnace control valves are initially in positions asfolnace Stop relay on line 23 f FIG 3 i d i d d lows: its contactsreverse position. At this instance the fur- VLIIVC Al Flue Air (Openswhen energized) Open nace start sequence begins since timer motor M(line Valve Aiil- Zonle l, Temp. Clontrjol (This: valve is Closed 22)runs h h Contact s 8, to Operate ti contro lngw en energize an is set orf minimum fire when deenergized). contacts 8-1 th-rough S 8' v e i P- 52 l g l 10 Zone 1 l 8 The furnace light-off sequence IS as follows (thetime alve itrogen quenc ottom o urnace ose Zone l) (opened only on linestop for approxcycle 5 Selecte'd as an mpi y and vanes wlth imatelythree niinutes) Furnace and line characteristics), see FIG. 3.

Time Contacts Function 0 Sequence timer Motor M starts when relay F.S.R.is deenergized and its contact on line 22 closes. 5 seconds S-l on Line2 Valve Al Closes.

opens. 10 seconds 5-2 on Line 13 Valve B3 closes and Valve Cl is opens.energized and opens. (Switch b-3 of valve B3, line 14 closes) (Zone 2lights on minimum). 12 seconds 5-3 on Line 4 Valve A5 opens. (Zone 2goes closes. on temperature control) 14 seconds 5-4 on Line l0 Valves BIand B2 open.

closes (Zone 1 lights on minimum). l5 seconds S-5 on Line 8 Timer TD 2is reset.

opens 16 seconds 5-6 on Line 3 Valve A2 opens. (Zone I goes on closestemperature control) 7 58 seconds 5-8 on Line 22 Timer is stopped.

opens. 3-7 on Line 2l closes.

In shutting down the furnace, the line may be stopped at the option ofthe line operator by pressing the Line Stop push button on line 17. Whenthe Line Stop push button on line 17 is pressed its contact is closedand is maintained closed, this being a conventional type of maintainedcontact which remains closed until it is manually opened. Time DelayRelays TD3F on line 16 and TD3S on line 16A are energized and begintiming. Relay RC is energized and its contact in line 6 closes,energizing nitrogen purge valves A3 and A4 on lines 6 and 7. The purgetimer on line 5 is also energized. When timer TD3F times out(approximately 5 seconds) its contact on line 24 closes and the furnaceStop Relay F.S.R. on line 23 is energized to initiate the furnace stopsequence. When timer TD3S times out (approximately l seconds) itscontact on line 19 opens to deenergize the Line Stop Relay L.S.R. online 19 to stop strip movement.

On line stop, the furnace start sequence timer motor M resets by runningfor two seconds and returns all start contacts S-l -8 to their initialpositions, as heretofore outlined.

The timer TDl, which was energized when the relay CR contact closed, maybe set for approximately a period of 3 minutes, after which it graduallycloses valves A3 and A4, shutting off the nitrogen quench.

After a delay of about 3 to 6 seconds, for example, depending on thespeed at which the temperature control valves move to their minimumpositions, valve Al, controlling the air bleed to the flue 6, has fullyopened and the furnace pressure control damper 21 moves under theinfluence of the furnace pressure controller to its new position tomaintain a positive pressure. Time delay relay TD2 operates to shut offthe air and gas to Zone 1 completely with gradual movement of valves B1and B2, and gradually to open the nitrogen valve B3. When this valve B3is fully open, a limit switch opens to cause the combustible mixturesupply valve C1 to close. The introduction of nitrogen into the Zone 2burner nozzles serves to maintain a burner pressure to prevent backfire,and to keep the burner tips from overheating.

LlNE STOP BY TEMPERATURE CONTROL While the line is operating atproduction speeds the strip may be overheated due to furnace malfunctionor due to some physical characteristics of the strip. Should overheatingoccur at these speeds, a contact (line in the temperature controllercloses and actuates timers TD4F on line 17 and TD4S on line 17(0), andthe alarm shown in line 18. The line operator is now alerted and musttry to correct the condition operationally. If the temperature is notcorrected within one minute, the

timer contact TD4F on line 25 closes to energize the furnace stop relayF.S.R. on line 23 and initiate furnace shutdown. Five seconds later,TD4S contact on line 19 opens to stop strip movement.

When the line is being slowed without the operator pressing the stopbutton, and after closing of the minimum process speed contact MSCheretofore described, (see Line 15 in FIG. 3), the strip will overheatat some low speed. In this case, timer TD3F on line 16, timer TD3S online 16(a) and a relay RC on line 15 are energized. The relay contact(line 6) of relay RC closes to energize valves A3 and A4, thus admittingnitrogen quench to the furnace, A-3 at the bottom of Zone 1 and A-4 atthe bottom of Zone 1. Time delay relay TD3F is set to delay for 5seconds, for example, and then shuts down the furnace by energizingrelay F.S.R. on line 23. Also, relay TD3S contact in line 19 opens 5seconds later, deenergizing relay L.S.R. on line 19 to stop stripmovement. The time of shutting down the furnace after pressing the stopbutton and the further time delay for strip travel are adjustable bysimply adjusting the settings of time delay relays TD3F, TD3S, TD4F andTD4S.

It will be appreciated that the valves that are used in controlling theflow rates of fuel and air into the burners, and of protective gas intothe zones of the furnace chamber, are carefully constructed so that theydo not open or close instantaneously. On the contrary, means aredeliberately introduced into the valves to limit the rate at which theycan open or close. In a conventional air operated diaphragm type valve,for example, the speed of operation of the valve is limited by insertingan orifice plate of predetermined size into the air line which leads tothe diaphragm chamber.

Accordingly, when reference is made herein to any of the aforementionedcontrol valves opening or closing, it should be appreciated that suchaction takes place slowly and gradually over a period of 2 to 5 seconds,more or less, for example. This gradual adjustment of a valve from aclosed to an open position provides, a flow rate which graduallyincreases or gradually decreases over a predetermined period of time.

For example, when the furnace is being shut down due to overheating ofthe strip, the valves A2 and A5 that control fuel and air to the burnersmove gradually toward thier minimum fire positions, causing a gradualreduction of feed rate for a considerable period of time (2-5 seconds,for example). Similarly, the valves A3 and A4 controlling theintroduction of cooling gas (such as N or HNX, for example) opengradually and slowly, gradually increasing the feed rate of the gas fora considerable period of time (2-5 seconds, for example).

In a shutdown, the introduction of cooling gas and the shutting off ofburner fuel and air do not occur simultaneously. The rate of flow offuel and air is gradually decreased to a predetermined low rate. Theprotective gas flow rate gradually increases until it reaches itsmaximum. Steady flow of all materials follows. Then, later on the fueland air are shut off.

It will be appreciated that various changes may be made in the form ofthe invention shown in the drawings and described herein. If desired,the protective gas may be introduced at various periods of time beforethe fuel or air valves are actuated. Further, the rates of movement ofthe control valves may be varied, provided the pressure in the furnacechamber is maintained above atmospheric pressure. Other changes in thespecific form, arrangement and sequencing of the electrical and controlelements shown in FIGS. 2 and 3 may be made, and equivalent elements maybe substituted, parts may be reversed, and certain features may be usedindependently of others, all without departing from the spirit and scopeof this invention as defined in the appended claims.

The following is claimed:

1. In a method of preventing oxidation of a metal strip in shutting downa furnace chamber having a plurality of burners, the steps whichcomprise:

a. reducing the fuel supply to the burners from its normal supply rateto a low fuel supply rate which is much lower than said normal rate butsubstantially above zero,

b. maintaining said low fuel supply rate substantially constant for aperiod of time,

c. introducing'into said chamber a protective gas which is non-oxidizingto the metal strip,

d. maintaining said feed of said protective gas and concurrentlymaintaining said fuel at said low fuel supply rate, both for apredetermined period of time, and

e. thereafter shutting off said fuel supply.

2. The method defined in claim 1 wherein positive pressure is maintainedwithin the chamber throughout steps (a) through (e).

3. The method defined in claim 1 wherein steps (a) through (e) areautomatically actuated in response to sensing the strip temperature, thetemperature of actuation being above a predetermined temperature.

4. The method defined in claim 3, wherein the strip temperature issensed at a location in the vicinity of the exit of the strip from thefurnace chamber.

5. The method defined in claim 4, wherein the furnace chamber is directfired and the strip temperature is sensed after the strip has exitedfrom the direct fired portion of the furnace chamber.

6. The method defined in claim 1, wherein step (c) occurs prior to step(a).

7. The method defined in claim 1, wherein a time delay is provided afterany of steps (a) (e) following which the strip movement is stopped.

8. In the method of claim 1, wherein said chamber includes a passagewayfor exhaust gases, which passageway has an opening provided with acontrollable closure for said opening, the step which comprises closingsaid opening during the introduction of said protective gas into saidchamber.

9. The method defined in claim 1, wherein said chamber has adjacentzones one above the other, and wherein protective gas is separatelyintroduced into each of said zones.

10. The method defined in claim 1, wherein the protective gas isintroduced into said chamber at a gradu- 10 ally increasing rate over apredetermined period of time.

11. In the method of claim 1, wherein said furnace chamber has an outletthat is in communication with a strip treating chamber containing aprotective gas and also having openings to the atmosphere, the step ofmaintaining the pressure in said furnace chamber higher thanthe'pressure in said strip treating chamber to prevent any substantialinward flow of said atmosphere into the protective gas in said chamber.

12. In an apparatus for heat treating metal strip, which apparatusincludes a furnace having a chamber containing a plurality of burners,the fuel feed to which is valve controlled, means for passing said stripthrough said furnace, and means for introducing a protective gas intothe furnace chamber, an automatic sequence control having a turningdevice connected to actuate said control valve for reducing the rate offuel to the burners, time delay means and means for introducing saidprotective gas into said furnace chamber after a predetermined timedelay, and in response to said time delay means.

13. The apparatus defined in claim 12 wherein means are provided formaintaining positive pressure in said chamber when the rate of feed offuel is reduced.

14. The apparatus defined in claim 12, wherein a temperature sensingmeans is arranged in the vicinity of the strip exit from the chamber,and wherein said temperature sensing means is calibrated to actuate saidcontrol valve to initiate a reduction of said rate of feed of fuel.

15. The apparatus defined in claim 12, wherein said chamber has a fluefor exhaust gases, provided with a controllable flue damper whereinmeans are provided for signalling a furnace shutdown, and whereincontrol means is provided for closing the flue damper in response to afurnace shutdown signal.

16. The apparatus defined in claim 12, wherein said chamber has adjacentzones one above the other, and wherein separate means are provided forintroducing protective gas into each of said zones.

17. The apparatus defined in claim 12, wherein said furnace chamber hasan outlet that is in communication with a strip treating chambercontaining a protective gas and also having openings to the atmosphere,said protective gas introducing means having sufficient capacity, aftersaid time delay means has operated, to maintain a pressure within saidfurnace chamber which is greater than the gas pressure within said striptreating chamber.

18. The method defined in claim 1 wherein concurrently with the furnaceshutdown sequence, air is introduced into the flue of said furnacechamber.

19. The method defined in claim 1 wherein air is introduced in aselected area of the furnace chamber to provide continuing cooling.

UNITED STATES PATENT OFFICE I CERTIFICATE OF CORRECTION Patent 3.827.854AUGUsT 6, 1974 Inventor WILLIAM E GILDERSLEEVE It is certified thaterror appears in the aboveidentified paterlt and that said LettersPatent are hereby corrected as shown below:

SELAS CORPORATION OF AMERICA .[73] Assignee:

' Dresher, Pennsylvania to be added to the Title Page.

Signed and sealed this 3rd day of December 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. A c." MARSHALL DANN Attesting Officer Commissionerof Patents USCOMM-DC 60376-P69 us. GOVERNMENT PRINTING omcs: I969o-ass-su FORM Po-1 050 (10-69)

1. In a method of preventing oxidation of a metal strip in shutting downa furnace chamber having a plurality of burners, the steps whichcomprise: a. reducing the fuel supply to the burners from its normalsupply rate to a low fuel supply rate which is much lower than saidnormal rate but substantially above zero, b. maintaining said low fuelsupply rate substantially constant for a period of time, c. introducinginto said chamber a protective gas which is nonoxidizing to the metalstrip, d. maintaining said feed of said protective gas and concurrentlymaintaining said fuel at said low fuel supply rate, both for apredetermined period of time, and e. thereafter shutting off said fuelsupply.
 2. The method defined in claim 1 wherein positive pressure ismaintained within the chamber throughout steps (a) through (e).
 3. Themethod defined in claim 1 wherein steps (a) through (e) areautomatically actuated in response to sensing the strip temperature, thetemperature of actuation being above a predetermined temperature.
 4. Themethod defined in claim 3, wherein the strip temperature is sensed at alocation in the vicinity of the exit of the strip from the furnacechamber.
 5. The method defined in claim 4, wherein the furnace chamberis direct fired and the strip temperature is sensed after the strip hasexited from the direct fired portion of the furnace chamber.
 6. Themethod defined in claim 1, wherein step (c) occurs prior to step (a). 7.The method defined in claim 1, wherein a time delay is provided afterany of steps (a) - (e) following which the strip movement is stopped. 8.In the method of claim 1, wherein said chamber includes a passageway forexhaust gases, which passageway has an opening provided with acontrollable closure for said opening, the step which comprises closingsaid opening during the introduction of said protective gas into saidchamber.
 9. The method defined in claim 1, wherein said chamber hasadjacent zones one above the other, and wherein protective gas isseparately introduced into each of said zones.
 10. The method defined inclaim 1, wherein the protective gas is introduced into said chamber at agradually increasing rate over a predetermined period of time.
 11. Inthe method of claim 1, wherein said furnace chamber has an outlet thatis in communication with a strip treating chamber containing aprotective gas and also having openings to the atmosphere, the step ofmaintaining the pressure in said furnace chamber higher than thepressure in said strip treating chamber to prevent any substantialinward flow of said atmosphere into the protective gas in said chamber.12. In an apparatus for heat treating metal strip, which apparatusincludes a furnace having a chamber containing a plurality of burners,the fuel feed to which is valve controlled, means for passing said stripthrough said furnace, and means for introducing a protective gas intothe furnace chamber, an automatic sequence control having a turningdevice connected to actuate said control valve for reducing the rate offuel to the burners, time delay means and means for introducing saidprotective gas into said furnace chamber after a predetermined timedelay, and in response to said time delay means.
 13. The apparatusdefined in claim 12 wherein means are provided for maintaining positivepressure in said chamber when the rate of feed of fuel is reduced. 14.The apparatus defined in claim 12, wherein a temperature sensing meansis arranged in the vicinity of the strip exit from the chamber, andwherein said temperature sensing means is calibrated to actuate saidcontrol valve to initiate a reduction of said rate of feed of fuel. 15.The apparatus defined in claim 12, wherein said chamber has a flue forexhaust gases, provided with a controllable flue damper wherein meansare provided for signalling a furnace shutdown, and wherein controlmeans is provided for closing the flue damper in response to a furnaceshutdown signal.
 16. The apparatus defined in claim 12, wherein saidchamber has adjacent zones one above the other, and wherein separatemeans are provided for introducing protective gas into each of saidzones.
 17. The apparatus defined in claim 12, wherein said furnacechamber has an outlet that is in communication with a strip treatingchamber containing a protective gas and also having openings to theatmosphere, said protective gas introducing means having sufficientcapacity, after said time delay means has operated, to maintain apressure within said furnace chamber which is greater than the gaspressure within said strip treating chamber.
 18. The method defined inclaim 1 wherein concurrently with the furnace shutdown sequence, air isintroduced into the flue of said furnace chamber.
 19. The method definedin claim 1 wherein air is introduced in a selected area of the furnacechamber to provide continuing cooling.